The present invention relates to vessels for containing mobile or highly permeating fluids and more particularly to such vessels which incorporate an improved barrier film.
Heretofore, cryogenic tank structures for storing spacecraft fuels, such as liquid hydrogen and methane, have typically been either entirely metallic or have employed a metal liner with a composite overwrap. These structures have not been entirely satisfactory in that the amount of metal employed, even as a liner, contributed inordinately to the weight of the spacecraft. As is understood, a certain minimum thickness of metal is required to order to permit welding for fabrication and this thickness is substantially greater than that thickness which would be necessary merely to provide the necessary impermeability. Further, tanks constructed from metal liners with composite overwraps have exhibited a relatively short life due to the mismatch in the coefficients of thermal expansion between the metal and the composite overwrap.
While composite tanks without liners have been considered, two problems have prevented the satisfactory development of such a tank: microcracking of the matrix and the relatively high gas permeability of the matrix. Microcracking of the composite is caused by the strain between the fiber bundles and the interlaminar resin. Leakage from the tanks occurs due to the porosity which is essentially inevitable in the usual method of construction which is filament winding. The permeability of most resin and polymer materials creates a similar problem with respect to this construction of scientific research balloon, particularly those intended for very high altitudes or very long duration flights. The problem with permeability is particularly acute with so-called over-pressure balloon structures in which the pressure in the interior of the balloon is appreciably higher than that of the atmosphere in which the balloon is traveling.
The improvements which are obtained with the vessel structures of the present invention are predicated upon the unexpected discovery that certain ordered liquid crystal polymers are surprisingly impermeable. Polybenzazole (PBZ) polymers are preferred liquid crystal polymers for use in the present invention. Preferred PBZ polymers are selected from the group consisting of polybenzoxazole (PBO), polybenzothiazole (PBT or PBZT) and polybenzimidazole (PBI) polymers, and random, sequential or block copolymers thereof. Polybenzazole polymers and their synthesis are described at length in numberous references, such as Wolfe, Liquid Crystalline Polymer Compositions, Process and Products, U.S. Pat. No. 4,533,693 (Aug. 6, 1985) and W. W. Adams et al., The Material Science and Engineering of Rigid-Rod Polymers (Materials Research Society 1989), which are incorporated herein by reference. Such polymers are also disclosed in commonly assigned, copending application U.S. Ser. Nos. 07/098,710, filed Sep. 21, 1987, now U.S. Pat. No. 4,973,442, and 07/206,137, filed Jun. 13, 1988, now U.S. Pat. No. 4,963,428, which are incorporated herein by reference.
PBZ polymers preferably contain a plurality of mer units that are AB-PBZ mer units, as represented in Formula 1(a), and/or AA/BB-PBZ mer units, as represented in Formula 1(b) ##STR1## wherein: Each Ar represents an aromatic group. The aromatic group may be heterocyclic, such as a pyridinylene group, but it is preferably carbocyclic. The aromatic group may be a fused or unfused polycyclic system. The aromatic group preferably contains no more than about three six-membered rings, more preferably contains no more than about two six-membered rings and most preferably consists essentially of a single six-membered ring. Examples of suitable aromatic groups include phenylene moieties, biphenylene moieties and bisphenylene ether moieties. Each Ar is most preferably a 1,2,4,5-phenylene moiety.
Each Z is independently an oxygen atom, a sulfur atom or a nitrogen atom bonded to an alkyl group or a hydrogen atom. Each Z is preferably oxygen or sulfur (the polymer is preferably PBO, PBZT or a copolymer thereof); PA1 Each DM is independently a bond or a divalent organic moiety that does not interfere with the synthesis, fabrication or use of the polymer. The divalent organic moiety may contain an aliphatic group (preferably C.sub.1 to C.sub.12), but the divalent organic moiety is preferably an aromatic group (Ar) as previously described. Each DM is preferably a 1,4-phenylene moiety or a 4,4'-biphenylene moiety, and is most preferably a 1,4-phenylene moiety. PA1 The nitrogen atom and the Z moiety in each azole ring are bonded to adjacent carbon atoms in the aromatic group, such that a five-membered azole ring fused with the aromatic group is formed.
The azole rings in AA/BB-PBZ mer units may be in cis- or trans-position with respect to each other, as illustrated in 11 Ency. Poly. Sci. & Eng., 601, at 602, (J. Wiley & Sons 1988) which is incorporated herein by reference.
The PBZ polymer may be rigid rod, semirigid rod or flexible coil. It is preferably rigid rod in the case of an AA/BB-PBZ polymer or semirigid in the case of an AB-PBZ polymer. It more preferably consists essentially of AA/BB-PBZ mer units. Exemplary highly preferred mer units are illustrated in Formulas 2 (a)-(e). ##STR2## The polybenzazole polymer most preferably consists essentialy either of the mer units illustrated Formula 2(a) (cis-PBO) or of the mer units illustrated in Formula 2(c) (trans-PBZT).
Each polymer preferably contains on average at least about 25 mer units, more preferably at least about 50 mer units and most preferably at least about 100 mer units. The intrinsic viscosity of cis-PBO or trans-PBZT in methanesulfonic acid at 25.degree. C. is preferably at least about 10 dL/g, more preferably at least about 20 dL/g and most preferably at least about 30 dL/g.
Unlike ordinary polymer molecules which are twisted and coiled, liquid crystal polymer molecules are rigid and rod-like. The micro structure consists of densely packed fibrous polymer chains or microfibrils. The microfibrils can be oriented through processing, e.g. by extruding the polymer through counter-rotating cylindrical dies followed by blowing and drawing, so as to yield a biaxial orientation. For example, PBZT resin is a lyotropic liquid crystal polymer and the extrusion is typically performed into a bath of polyphosphoric acid or similar solvent so that the extrusion results in a water-swollen oriented film. After the forming steps, the film is typically washed, dried and rolled under controlled pressure to remove the solvent and densify the sheet. Given the microfibrillar nature of the film microstructure, it was expected in accordance with conventional wisdom that these films would be highly permeable and would be suitable for use as microfiltration membranes. The vessel structures of the present invention are based in substantial part on the unexpected discovery that films comprising lyotropic liquid crystal polymers, such as PBZ polymers, are highly impermeable. Further, such films exhibit very high strength and toughness and these characteristics can be utilized to augment the structural properties of a composite structure while at the same time providing the barrier characteristics which are necessary in certain vessel applications.
Among the several objects of the present invention may be noted the provision of a vessel which is effective for containing a mobile or highly permeating fluid; the provision of such a vessel which provides very low leakage rates; the provision of such a vessel which is lightweight and strong; the provision of such a vessel which can withstand repeated and extreme variations in temperature; the provision of such a vessel which is highly reliable and which is of relatively simple and inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.